1. Field of the Invention
The present invention relates to a process for production of rubber-modified styrene-based resins. More particularly, it is concerned with a process for producing rubber-modified styrene-based resins which are excellent in a physical balance; for example, have a high impact strength, and further excellent gloss and stiffness, and thus which are suitable as materials for production of office automation (OA) devices, home electric appliances, car parts, and so forth.
2. Background Information
Rubber-modified styrene-based resins are widely used in various fields because they are excellent in impact resistance and stiffness, and have good moldability. As these rubber-modified styrene-based resins, polybutadiene rubber and styrene-butadiene copolymer rubber have heretofore been generally used.
In order to exhibit the excellent physical properties of gloss and impact strength of rubber-modified styrene-based resins, it is necessary to control the rubber particle diameter to the optimum range and to provide a narrow particle size distribution. In the case of polybutadiene rubber, since a phase transition (transition of a rubber phase from a continuous phase to a dispersion phase) at which rubber particles are formed occurs in an unstable region in the initial stage of polymerization at which the conversion of monomer is 5 to 15%, uniform mixing is needed, and in order to exhibit the gloss, it is necessary to control the rubber particle diameter to 0.5 to 1.5 .mu.m. For these reasons, it is necessary to employ a full mixing vessel-type reactor which is able to provide a high shearing force.
Use of such full mixing vessel-type reactors, however, has disadvantages in that the rubber particle size distribution is broadened and it is difficult to maintain a balance between gloss and impact strength. Thus it is considered that the phase transition in a system using polybutadiene rubber is caused by the use of a column-type reactor having a high plug flow, for example. In this case, however, high speed agitation providing a high shearing force is needed. In this agitation, it is required that in order to increase heat removal efficiency, a multi-plate reactor is used, and a heat conducting tube is passed through plates. Thus the above high speed agitation has disadvantages in that vibration of a stirring blade readily occurs, it is therefore necessary to increase the strength of the heat conducting tube, and in that a power neck is readily produced because of a high viscosity system.
In order to solve the above problems, it is considered to use a styrene-butadiene block copolymer rubber having a styrene content of at least 50% as used in Japanese Patent Publication No. 57433/1985.
This method, however, has disadvantages in that an interface tension with a polystyrene matrix is excessively decreased and thus even under a low shearing force, the rubber particle size is decreased, and the impact strength is extremely low although the gloss is very good.
In order to optimize the particle diameter in the above system, it is necessary to operate a column-type reactor while agitating at a very low speed. In this case, however, problems arise in removal of polymerization heat and insufficient agitation.